A Jaw Smaller Than a Thumbnail Is Rewriting Lizard Prehistory

A Fossil the Size of a Fingernail

It is a partial left lower jaw measuring less than 9 millimeters long — yet despite its tiny size, it preserves 12 teeth and evidence that the animal would have carried as many as 18 teeth in its dentary. This is the remarkable fossil at the center of a new study that is helping scientists fill one of paleontology's most stubborn gaps: the murky origins of the world's lizards, snakes, and their kin.

Today, lepidosaurs — the reptile group that includes lizards, snakes, and New Zealand's tuatara — are among the most diverse vertebrates on Earth, but their earliest evolutionary history remains poorly understood. A newly discovered fossil jaw from southern Brazil is helping paleontologists piece together the anatomy and evolutionary history of *Cargninia enigmatica*, a tiny lepidosaur relative that lived during the Late Triassic epoch alongside some of the earliest dinosaurs.

Life at the Dawn of the Dinosaur Age

*Cargninia enigmatica* roamed our planet during the early Norian age of the Late Triassic, around 225 million years ago. At the time, the animal shared its environment with early dinosaurs, mammal relatives, primitive crocodile-line reptiles, and several early relatives of the tuatara. Picture a world just beginning to fill with the creatures we associate with the Mesozoic — and scurrying through that landscape, a reptile so small it could fit in the palm of a human hand.

The new specimen was recovered from the type locality, the Linha São Luiz site, located in the municipality of Faxinal do Soturno, Rio Grande do Sul State, Brazil. In Brazilian strata, *Cargninia enigmatica* is the only representative of its lineage, and the taxon was found in Norian beds of the upper portion of the Candelária Sequence. Its rarity makes every new fragment of bone precious.

High-Tech Tools Unlock Ancient Secrets

Paleontologists employed micro-CT scans to analyze the internal structures of the specimen, allowing them to trace the trigeminal nerve responsible for facial and jaw sensation and motor control. This kind of neurological detail from a 225-million-year-old fragment is extraordinary — it means researchers can begin to understand not just what this creature looked like, but how it sensed its world.

The researchers concluded that "*Cargninia enigmatica* likely perceived its environment, at least with respect to trigeminal sensory inputs, in a manner comparable to that of its extant relatives." In other words, the fundamental sensory wiring that modern lizards rely on today may have roots stretching back to the very dawn of the group's existence.

The scientists also ran *Cargninia enigmatica* through a large computational phylogenetic analysis. The species consistently emerged as a non-lepidosaur lepidosauromorph, supporting the idea that it represents an early branch that split off before true lepidosaurs evolved. "Its phylogenetic position was investigated in a computational context for the first time, and the species was recovered among non-lepidosaur lepidosauromorphs, corroborating previous assumptions," the authors said.

Why a Tiny Jaw Matters So Much

The new study describes fossil material of *Cargninia enigmatica* from the Norian strata of the Candelária Sequence of the Santa Maria Supersequence, providing new information on the anatomy of the dentary and on the neuroanatomy of members of this species. That dual contribution — both skeletal anatomy and neurological structure — makes this find unusually rich for such a small specimen.

Broad comparisons suggest that *C. enigmatica* possesses a unique combination of features, but several key features are widely distributed among basal non-rhynchocephalian lepidosauromorphs and kuehneosaurids. Two possible scenarios have been proposed: that *Cargninia* is a basal non-lepidosaurian lepidosauromorph, or that it is a basal lepidosaurian. Resolving that question will require more fossil material — but each new scrap of bone narrows the uncertainty.

The study, authored by Lísie Vitória Soares Damke and colleagues, was published in *The Anatomical Record*. It is a reminder that some of the most consequential discoveries in science come not from dramatic, complete skeletons, but from fragments so small they could be mistaken for pebbles. As researchers continue excavating Brazil's Triassic beds, the picture of how today's 10,000-plus species of lizards and snakes came to conquer the planet is slowly, painstakingly, coming into focus.